One of the goals of behavioral biologists is to understand why sometimes there is close cooperation among in animal societies and why at other times there is intense conflict. This project will help us understand the shifting balance between cooperation and conflict by investigating a striking switch from strong cooperation to stunning conflict inside the nests of a common species of yellow jacket wasp called Dolichovespula arenaria. The nests of these wasps are the familiar grey, paper structures about the size of a soccer ball that appear in trees and on buildings. Inside each one lives a family of wasps that includes a mother wasp (the queen) and dozens of her daughters and sons. Most of the time, the daughter wasps cooperate with their mother, helping her by building the nest, defending it, collecting food, and helping rear the queen's offspring. But late in the summer, one of the daughter wasps will suddenly sting and kill the mother wasp and then start laying eggs herself. How such a dramatic switch from cooperation to conflict can have evolved is the puzzle that this study addresses. Using novel video recording methods that enable us to carefully monitor the situation inside each colony at the time of the matricide, the PI will test several hypotheses for how it can actually benefit a daughter wasp to kill her mother. One possibility is that the mother wasp has lost her fertility, in which case the workers have little to lose and much to gain by killing their mother and taking over the nest. This work will be deepen our understanding of the biological forces that influence cooperation and conflict, and this is a matter of great relevance to human society.
In this project, we addressed the question of why workers in some colonies of yellowjacket social wasps appear to revolt, stinging the queen, their own mother, to death. Such behavior is an evolutionary puzzle, because workers in this species cannot rear a new queen and may thus pay a large fitness cost by eliminating the only source of new queens and more workers. The hypothesized benefits to workers are that they may be able to rear more sons of their own (workers can lay male, but not female, eggs) as well as more sons of other workers, which they value more than the sons of the queen. If matricide behavior is truly driven by conflict over the production of males, we can make several predictions based on simple evolutionary models. One is that workers should be sensitive to the sex ratio of the offspring the queen is producing: if she is producing males but not females, matricide is most beneficial. Another scenario in which workers would be most benefited by matricide is if workers who kill the queen are not daughters from that colony, but are instead invaders from a nearby colony. These workers would have none of the costs of killing the queen, but would still benefit from being able to lay their own eggs in the colony. We tested these hypotheses, as well as several others, by observing colonies of yellowjackets in glass-walled boxes (see attached image). We documented queen-killing events with continuously recording cameras and used genetic markers to determine if foreign workers were present in colonies in which the queen died. Finally, to test the hypothesis that queens laying only male eggs are especially vulnerable to matricide, we performed surgery on queens to remove their sperm-storage organ. This, due to their haplodiploid genetic system, forces queens to lay only male eggs. We predicted that these manipulated queens should experience matricide more frequently than sham-surgery queens that were cut open but did not have their sperm-storage organ removed. Our results were quite clear. We observed matricide in three colonies out of 21, suggesting it may be rarer than previously reported. In these colonies, no foreign workers were detected with our genetic analyses, so queen killing behavior definitely involves daughters killing their mother. In neither our experimental group of male-laying queens, nor our group of sham-surgery queens, did workers kill the queen. This was the case despite the surgery-queens laying eggs and surviving for 28 days, enough time for those eggs to develop into adults. Thus, we can conclude that a male-biased sex ratio of the brood is not sufficient to induce matricide behavior in these wasps. Our results thus suggest that matricide is due to conflict among family members over reproduction, but the factors that trigger the behavior remain a mystery.
